1. Field of the Invention
The present invention relates to zener diodes and, more particularly, to zener diodes adapted for mounting in packages that require soldered connections to the diode device .
2. Description of the Prior Art
In applicant's copending application, Ser. No. 557,206 filed Mar. 10, 1975 entitled Zener Diode and issued as U.S. Pat. No. 4,030,117 on June 14, 1977 a zener diode is disclosed that is uniquely applicable for use in a double plug package that is inexpensive and may be machine assembled. The zener diode disclosed in the copending application had many advantages and overcame many difficulties, such as an unacceptable reject rate resulting from shorted junctions caused by alloying of contact metal into the semiconductive material during the high temperature sealing operation required for double plug packages. The disclosed zener diode also overcame poor yields and unpredictable diode characteristics that resulted from the pressure exerted on the silicon material during packaging of the zener diode in the double plug package.
In the copending application, it was taught that by positioning the contact over the deep diffused region, the premature breakdown caused by surface imperfections was eliminated since avalanching did not as readily occur in the deep junction region where the breakdown voltage was substantially higher than in the shallow junction region. It was also determined that by contacting the deep diffused region over its entire length, the current was more advantageously distributed to prevent high current density and the tendency toward avalanche at high current density points. The force exerted on the contact during double plug packaging was not as detrimental to the diode device when the contact was made to the deep diffused region as opposed to the shallow junction region and the force was spread out over a larger contact area thereby reducing the pressure on the contact.
While the zener diode taught in the copending application had many advantages over the prior art, it could not be readily used with other types of packages where leads had to be soldered directly to the diode contacts. Difficulty was experienced in attempting to solder to the silver contact ring and in many instances, the ring cracked causing the solder to flow into the junction region resulting in an unacceptable device. Difficulty was also experienced when attempting to provide a larger area contact since the contact metal would not adhere well to the oxide layer and a hermetic seal could not be provided necessitating the use of an expensive hermetically sealed package.
Thus, the advantages of the zener diode taught in the copending application could not be realized in other types of packages requiring solderable connections.